(1) Field of the Invention
The present invention relates to an antenna suitable for use in a radio terminal and, more particularly, to a slot antenna using a coaxial cavity, a method of manufacturing such a slot antenna, and a radio terminal incorporating such a slot antenna.
(2) Description of the Prior Art
In radio terminals such as a personal handy phone system (PHS), the antenna of built-in type is increasingly used to enhance portability. Conventional antennas that can be built in radio terminals include the inverted F antenna such as disclosed in Japanese Patent Laid-open (Kokai) No. Hei 6-177629 and the microstrip antenna such as disclosed in Japanese Patent Laid-open (Kokai) No. Hei 5-327331. However, to allocate an enough band, it is required to make the dimensions of an antenna large, thereby making it difficult to build the antenna in a small-sized radio terminal. Besides, even if the antenna can be built in the radio terminal, an induced current is generated on the casing of the radio terminal by the radiation of electromagnetic wave emitted from the antenna, part of the generated induced current flowing in the operator's hand.
To solve such problems, a slot antenna using a coaxial cavity that operates in the TEM (Transverse Electromagnetic) mode was proposed (refer to Japanese Patent Laid-open (Kokai) No. Hei 5-14047 for example). Such a prior-art coaxial resonant slot antenna is shown in FIGS. 27A and 27B. FIG. 27A is a perspective view of the conventional antenna while FIG. 27B is a cross section along line 27B--27B of FIG. 27A. In the prior-art coaxial resonant slot antenna, a long strip conductor 53 is arranged along the resonant axis in the inner space of a conductive cubic 51 which is a rectangular prism in its entirety. Both ends 57 of the strip conductor are fixed on side walls 54 of the conductive cubic 51 in an electrically contact manner (refer to the cross section of FIG. 27B). The conductive cubic 51 is formed on one surface (an electromagnetic wave radiation surface) thereof with a slot 52 which crosses one end of the strip conductor 53. Further, an additional conductor 55 of center conductor of coaxial line is arranged along the length (the resonant axis) of the strip conductor 53 on an inner wall of the conductive cubic 51 at the other end of the strip conductor 53. It should be note that the conductive cubic 51 is maintained at ground potential by an outer conductor 56.
The conductive cubic 51 and the strip conductor 53 form, in cooperation, a coaxial resonant circuit in the TEM mode. Therefore, an electromagnetic wave (a transmission signal) supplied into the conductive cubic 51 via the additional conductor 55 progresses along the strip conductor 53 to be radiated into outside space via the slot 52. On the other hand, an electromagnetic wave (a receiving signal) entered in the conductive cubic 51 via the slot 55 from outside space progresses in the opposite direction along the strip conductor 53 to be picked up by the additional conductor 55.
Because the conventional coaxial resonant slot antenna has the strip conductor 53 with both ends 57 thereof electrically connected to the conductor cubic 51, it is necessary to set the total length of the antenna to 1/2 of a wavelength in use. Only when this setting is made, a value obtained by doubling the length of the conductive cubic 51 (or the length of the strip conductor 53) provides a resonant wavelength in the TEM mode, so that the intensity of electric field (amplitude) of the electromagnetic wave progressing through the conductive cubic becomes zero at the both ends 57 of the strip conductor 53 and reaches a maximum level at the center of the strip conductor 53. This denotes that it is impossible in principle to build the conventional coaxial resonant slot antenna in a radio terminal of which length along resonant axis is equivalent to less than 1/2 of wavelength in use. For example, at a frequency of 1.9 GHz used in the PHS, the 1/2 wavelength is about 80 mm, thus making it impossible to reduce the dimensions of radio terminals using such a frequency to less than 80 mm. In other words, if the conventional coaxial resonant slot antenna shown in FIG. 27 is built in a radio terminal, the dimensions of such a radio terminal cannot be reduced to less than 1/2 of the wavelength used.